The purpose of the proposed research is to develop computer algorithms for the accurate calculation of absorbed dose in cancer treatment with external photon beams. This will entail the development not only of accurate analytic methods of photon dose calculation, but also their implementation as computer algorithms in ways which provide calculation speed which is sufficiently high for routine clinical practice. The goal is to achieve in these computer algorithms an accuracy of 5%, or preferably 3%, throughout the treatment volume. For the proposed research, five dose-deposition processes for photon beams have been identified. Each of these processes will be modeled theoretically, and then practical computer algorithms will be developed for each one. (In research subsequent to this initial project period, computer algorithms are to be expanded so that they provide accurate dose calculation for realistic clinical treatment machines.) During the three years of proposed research, two theoretical physicists (with total time commitment of one full-time person) will be responsible for developing the five dose-deposition models from the basic laws of physics, and then mathematically simplifying them for implementation as computer algorithms by a full-time computer programmer. In addition, the computer programmer will continually carry out EGS4 Monte Carlo calculations to check the accuracy of the theoretical models and their attendant algorithms.